1. Field of the Invention
This invention relates to a roll used for guiding and running a steel sheet, or the like, among bath members arranged in a hot dipping apparatus, such as an immersion roll in a hot dipping bath (hereinafter referred to as a sink roll), and more particularly to an improvement in a surface structure of a sink roll applied to a hot dipping process of non-ferrous metal such as galvanization, aluminum hot dipping, zinc-aluminum hot dipping, tin hot dipping or the like.
2. Description of Background Material
The sink roll used in a galvanizing apparatus or aluminum hot dipping apparatus is immersed in a hot dipping bath as shown in FIG. 1. Since the sink roll is always used under the severe condition of immersion in molten metal, such as molten zinc or the like, it is fundamentally required to have the following functions:
(1) The roll surface is hardly eroded by molten metal;
(2) The roll surface is hardly abraded even in contact with a passing steel sheet and can maintain its initial shape accuracy for a long time; and
(3) Though the roll is a consumption article, the service life is long and the rise of cost in the apparatus can be suppressed.
Furthermore, the sink roll serves to guide the passing of the steel sheet and change the passing direction thereof in the hot dipping bath, so that the following functions are further required:
(4) The foreign matter suspended and floating in the hot dipping bath. ("dross", fine particles of Fe-Zn alloy or particles mechanically bonded with a metal component in the hot dipping, which has a melting point higher than that of metallic Zn and adheres to the surface of the steel sheet to cause a shape defect of a dipped layer and hence causes a surface defect of the product) hardly adheres to the steel sheet to be dipped; and
(5) In general, a helical groove for discharging the dross is formed on the outer surface of the sink roll, but the ununiform dipped portion, corresponding to the groove shape, is formed in the dipped steel sheet to cause surface defects such as uneven color tone, uneven gloss and the like (which is called a groove mark), so that it is required to prevent the occurrence of the surface defect even with use of a helical groove.
As the conventional sink roll satisfying the above requirements, there are (a) a sink roll provided at its outer peripheral surface with a coating layer having an excellent erosion resistance to molten metal, and (b) a sink roll provided at its outer peripheral surface with a groove having an improved shape. As the former roll, there are known (i) a spray-coating layer of cobalt self-fluxing alloy, as defined in JIS H8303-1989, (ii) a coating layer of Mo, W added to the above self-fluxing alloy for improving resistance to erosion of molten metal, as described in Japanese Patent laid open No. 1-108334, (iii) a coating layer of WC or CrC or TiC and a hot corrosion resistant metal having a thickness of 0.1-2.4 mm, as described in Japanese Patent Application Publication No. 58-37386, and the like.
As the latter roll, there is known (iv) a roll provided at its surface with plural grooves, as shown in FIGS. 2a and 2b. These grooves are a spiral groove formed in a unidirection toward a mother line of the roll or spiral grooves formed from a center of axis in opposite directions. A portion between the grooves has a flat shape. Further, there are proposed various methods of producing the spiral grooves, one of which is (v) a sink roll for hot dipping provided at its outer surface with a cross groove having a pitch of 20-60 mm, a depth of 0.5-10 mm, a width of 5-10 mm, and R of 3-10 mm as disclosed in Japanese Patent laid open No. 64-79356.
These conventional techniques are effective for the stabilization of the roll surface shape over a long period, reduction of dross adhered to the roll surface and the like in their own ways. However, the quality of the galvanization required in recent rust-preventive steel sheets for automobiles and the like has become considerably higher, and it is considered that the improvement of quality by the conventional techniques is insufficient. That is, it is actually demanded to improve the surface quality against uneven color tone, uneven gloss or the like even with the mechanical properties of the galvanized steel sheets, as well as the galvanized surface, as an undercoat for paint aimed at the invention.
Under such circumstances, the inventors have made various studies in order to improve the quality of the galvanized surface and confirmed that when the spiral grooves are formed on the surface of the roll, as described in Japanese Patent laid open No. 64-79356, to overcome various problems, uneven color tone or uneven gloss of the galvanized layer is caused by the spiral groove. That is, the steel sheet to be galvanized contacts the spiral grooves for the discharge of dross and the flat portions adjacent thereto, as shown in FIG. 2. The difference in the shape between the groove and the flat portion creates a strip pattern, called a "groove mark", on the galvanized steel sheet, which renders a defect in the galvanization appearance, such as uneven color tone, uneven gloss or the like in the galvanized surface of the steel sheet. Furthermore, it has been found that the groove mark remarkably occurs in a new product.
On the contrary, in order to prevent the occurrence of the groove mark, there are considered (1) a method of adjusting the tension of the steel sheet, the sheet passing velocity, the temperature of steel sheet, composition the of galvanizing bath and the like, and (2) a method of preliminarily passing a pretreated steel sheet, called as a dummy steel sheet, through a galvanizing bath for 8-24 hours to conduct conditioning and then actually passing a steel sheet to be galvanized therethrough. In the method (1), however, the thickness of the steel sheet to be produced, and the temperature and tension of the steel sheet in the hot-dipping bath, in accordance with the width thereof, change and hence it is difficult to find stable conditions. When the composition of the galvanizing bath is adjusted so as to make the groove mark unclear, such an adjustment is very difficult because it is conflicts with the adhesion property of the galvanized layer. On the other hand, the method (2) considerably lowers the productivity and hence is difficult to put into practical use.
From the above, it is clear that the formation of the spiral grooves through the conventional technique can not completely prevent the occurrence of the groove mark, even when conducting the improvement of any operations.